There are a number of industrial operations in which suspended particles must be separated from a liquid by settling, because the circumstances are not suitable for separation by screening, filtration or centrifuging. Separation by settling is usually the only practical expedient where the volume of liquid is large.
One major use for this type of equipment is in the handling of ash produced by solid fuel or refuse burning steam generating plants which, in the case of large plants, produce many tons of ash a day that is commonly transported in a water sluice conveyor to a dewatering tank or pond where the water is separated from the larger solid material and must then have the smaller particles removed by settling. The continuous production of ash in such plants requires a dynamic--i.e., continuous flow--system; and the distance of travel needed for continuous flow settling has resulted in such systems requiring an inordinate amount of space and construction material.
There have been systems in which the water-containing suspended particles have been introduced into the center of a large combination settling and surge or storage tank, with the particles settling in the center area of the tank and the relatively particle-free water flowing over a weir at the rim of the tank into an outflow channel. Such a unit, if it is to be adequate for the ash output from a single large furnace must be much larger in diameter than is desirable. Furthermore, like most settling units for the purpose, it must be elevated a substantial distance above ground level. Another common system utilizes a series of tanks with liquid flow from one to another and separate settling and storage. This requires a large head to sustain flow from one tank to another.